DVB-T2 as described in the DVB-T2 standard “Digital Video Broadcasting (DVB); frame structure channel coding and modulation for a second generation digital terrestrial television broadcasting system (DVB-T2)”, ETSI EN 302 755 V1.1.1 discloses a plurality of so-called “modcods”. A modcod is a pair consisting of a modulation/signal constellation such as QPSK, 16-/64-/256-QAM and code rates (1/2, 3/5, 2/3, 3/4, 4/5, 5/6). Each modcod has associated a spectral efficiency. The spectral efficiency is, for example, for a modcod of 16-QAM and a code rate of 2/3 as follows: 4 codebits/symbol*2 infobits/3 codebits=8/3 infobits/symbol=2.67 bits/s/Hz. Additionally, a constellation rotation including a coordinate interleaving can occur subsequent to the mapping of codebits. This procedure is, for example, disclosed in “Jonathan Stott: ‘Rotated Constellations’ available from http://www.dtg.org.uk/dtg/t2docs/RotCon_Jonathon_Stott_BBC.pdf”. The advantage of such a constellation rotation is a higher diversity when transmitting the coded signal which has been mapped to a certain signal constellation. This results in a higher robustness for a given modcod and a spectral efficiency provided by the given modcod.
The DVB-NGH (Next Generation Handheld) standard is very similar to the DVB-T2 standard and shares many of its blocks. Among others, it supports a large deal of the modcod parameters of DVB-T2, and it provides the option for constellation rotation and coordinate interleaving.
Typically, a DVB transmission comprises an FEC encoder for applying a certain forward error correction code to an information word. An information word may, for example, consist of 10,000 bits and advantageously consists of, for example, 1,000 bits to 100,000 bits. Depending on the code rate, the FEC encoder generates a codeword from the information word. When there is a code rate of, for example, 1/3, the codeword consists of 30,000 bits when the information word has 10,000 bits. For example, when the information word has 100,000 bits, then the codeword will have 300,000 bits. The bits of the codeword are introduced into a subsequent bit interleaver. The bit interleaver performs an interleaving within the codeword only, i.e. the for example 300,000 bits within an encoded codeword are interleaved so that an interleaved codeword results, but bits from one codeword are not interleaved with bits of a different codeword. Then, subsequent to the bit interleaver, an interleaved codeword having codebits exists. The codebits are grouped depending on a certain constellation diagram applied in a constellation mapping procedure. When the constellation diagram is, for example, a 256-QAM constellation diagram, then groups of 8 codebits are formed in order to map this group of 8 codebits into a constellation symbol. In 64-QAM, only 6 bits are grouped and mapped to one of the 64 different QAM symbols. Depending on the implementation, a constellation rotation and a cyclic Q-delay can be applied to the individual symbols in order to obtain individual cells. However, the constellation rotation or the cyclic Q-delay can be dispensed with so that the symbols output by the constellation mapping are the same as the so-called cells in the context of the DVB standard. Then, cells are input into a cell and time interleaver to obtain interleaved cells. The cell interleaver interleaves within the number of cells making up a certain codeword, but no interleaving within the cells/modulation symbols themselves occurs. In the time interleaver the number of cells making up a certain codeword are interleaved with cells from a different codeword, but no interleaving within the cells/modulation symbols themselves occurs. The individual modulation symbols are expressed as complex numbers, where each complex number has an in-phase component or I-component and quadrature component (O-component). A pair of an I-component and a Q-component which are also called “data units” makes up a constellation symbol or cell. However, with constellation rotation and cyclic Q-delay, a cell is different from a symbol in that a Q-component of a different symbol is paired with a I-component of another symbol while, without the constellation rotation or a cyclic Q-delay, the paired I-component and O-component of a cell actually make up the constellation symbol in the I/Q plan. Then, subsequent to the cell and time interleaver, the interleaved cells are forwarded to a frame builder, that produces the frames to be transmitted.
The FEC encoder performs a channel encoding. The bit interleaver is provided for destroying statistical dependencies which would be there in the receiver between the bits of a symbol, such as the 8 bits of a 256-QAM. These statistical dependencies would have a negative impact on the decoding of the channel codes. For example, when a 256-QAM-symbol would be heavily distorted, then 8 sequential bits would be non-decodable, and such a so-called burst error would result in a more negative impact when compared to a situation where the bit interleaving is applied.
The constellation is obtained, as discussed before, by a mapping of the codebits to a certain desired signal constellation such as 16-QAM.
The constellation rotation and cyclic Q-delay is optional. However, the following example clarifies the technology behind the cyclic Q-delay as described in the prior art reference mentioned before.
[Before any Cyclic Delay]
Cell1 I1Q1
Cell2 I2Q2
Cell3 I3Q3
Cell4 I4Q4
[After Cyclic Delay of Length=4]
Cell1 I2Q1
Cell2 I3Q2
Cell3 I4Q3
Cell4 I1Q4
The cell interleaver makes sure that the I and Q coordinates of a symbol are transmitted at different time instants and on different subcarriers of, for example, an OFDM signal (OFDM=orthogonal frequency division multiplex).
The time interleaver distributes the cells, which belong to an FEC codeword, over a certain time which is also called the interleaver time period. This provides time diversity. Time diversity is advantageous in that only a portion of an FEC codeword is strongly distorted when the transmission channel is not so good at a certain time instant. However, the remaining less distorted portion of the codeword might be sufficient for a successful decoding operation.
The frame builder builds the transmission frames, where a transmission frame defines the actual transmission signal for a predetermined time interval such as 200 ms. Since the T2 standard allows several physical layer pipes (PLPs), i.e. more parallel structures, but with individual modcods, the frame builder builds the frames from different output signals of several existing time interleavers. Such an individual processing chain is also called a “pipe” in the DVB context.
On the receiver side, the chain is processed in the reverse order. One of the blocks in the receiver is the time de-interleaver. The time de-interleaver operates in a cell-wise manner, wherein a cell can comprise, e.g., a received non-rotated QPSK or a rotated 256-QAM. A rotated 256-QAM has 256 possible values for the I-coordinate and additionally for the O-coordinate. This means that a cell can have values such as a (transmitted in a noisy channel) 256*256-QAM=65 k-QAM, where, by contrast to a conventional 65 k-QAM, the constellation point grid is non-regular. Since a cell can be any one of these constellations, it is necessary to finely quantize the I- and O-coordinates in the receiver before the I- and Q-coordinates are input into the time de-interleaver. In the DVB-T2-implementation guidelines: “Digital Video Broadcasting (DVB); Implementation guidelines for a second generation digital terrestrial television broadcasting system (DVB-T2)”, ETSI TR 102 831, it is outlined that one should apply a 10-bit quantization for the l- and Q-components and one should also provide several additional bits for the channel state information, i.e. for the information on an estimated signal-to-noise ratio (SNR) for this cell so that, in the end, one will necessitate 24 to 30 bits per cell, where a cell comprises a pair of data units, i.e. an I portion as a first data unit, a Q portion as the second data unit and the channel state information bits.
Subsequently, reference is made to FIG. 11 illustrating a certain portion of a DVB-T2-transmitter. The transmitter comprises an FEC encoder 1100. The output of the FEC encoder 1100 is connected to an input of a bit interleaver 1101. The FEC encoder 1100 receives, as an input, an information word which has, for example, 8100 bits and, provided that the FEC encoder 1100 applies an FEC code rate of 0.5, the number of code bits for an encoded information word or FEC codeword is 16,200 bits. This situation is illustrated schematically in FIG. 10 where item 1000 illustrates an information word and item 1001 illustrates an FEC codeword. The bit interleaver 1101 performs a bit interleaving within the bits of a single codeword and makes sure that bits of a codeword are not distributed into other codewords and vice versa. However, the bit order of the FEC encoder output is changed in order to increase the robustness for certain transmission conditions. The bit interleaver 1101 outputs code bits which are then input into a mapper performing a constellation mapping 1102. The mapping is, for example, a QPSK mapping where two bits are mapped into a single constellation symbol, so that the output of the constellation mapper 1102 is a group of 8100 symbols for the embodiment as illustrated at 1002 in FIG. 10. Then, the prior art provides for a constellation rotation, and cyclic Q delay operation in block 1103. The output of block 1103 is named a group of cells, where a cell consists of an I component and a Q component, but due to the cyclic Q delay, the Q component in a cell is different from the Q component which actually belongs to the I component in the cell as determined by the constellation mapper and as has been discussed before. This output of the cyclic Q delay is input to the cell interleaver in block 1104, where each input element consists of a first component or I component and a second component or Q component. When, block 1103 is not used, then a mapped constellation symbol is input into the cell interleaver in block 1104. The output of the cell interleaver becomes the input of the time interleaver 1105. The time interleaver 1105 finally outputs interleaved cells which are grouped into interleaving units and the interleaving units are supplied to a frame builder 1106 which then builds up the transmission frames.
The block constellation rotation and cyclic Q delay 1103 in FIG. 11 can be optional. Therefore, the input into the cell interleaver 1104 and time interleaver 1105 can be one of the following:
In a first possibility, the cells are normal signal constellations or mapped symbols and in the alternative possibility, the cells are rotated co-ordinate interleaved constellations which are rotated cells additionally including a cyclic delay as discussed before.
As already described in Jonathon Stott, “Rotated Constellations”, the cyclic Q delay and the cell interleaver ensure that the I- and Q component of a rotated (QAM-) symbol are transmitted at different times and/or different frequencies (i.e., different sub-carriers of an OFDM symbol). This is visualized in FIGS. 3A-3C. FIG. 3A illustrates a frame separated into different frequency sub-carriers, where an I component and a Q component are positioned at different frequency sub-carriers, but within the same time frame. FIG. 3B illustrates the situation where a time-interleaving has taken place and the I component and the Q component of one mapped symbol are transmitted in the same frequency sub-carriers, but at different time instants, i.e., in different frames. Finally, FIG. 3C illustrates the situation where the I component and the Q component of one and the same mapped symbol are transmitted at different times and different frequency sub-carriers.
The benefit of this is that the I- and O-components of a rotated (QAM-) symbol are attenuated differently in a time- and/or frequency-selective channel (i.e. a fading and dispersive multi-path channel like a Typical Urban channel with 6 paths—TU6). Hence one can achieve diversity within a (QAM-) symbol, which is not possible for conventional modulation (i.e. without constellation rotation and co-ordinate interleaving). Note that co-ordinate interleaving is realized in DVB-T2 by the Cyclic Q Delay and the Cell Interleaver.
FIGS. 4A-4F show this diversity effect exemplarily for a rotated QPSK constellation. The left-hand side shows a non-rotated constellation, where the I and Q components are necessarily attenuated in the same way. The right-hand side shows a rotated constellation after co-ordinate interleaving in the transmitter, individual fading of the I- and Q-components in the channel, and co-ordinate de-interleaving in the receiver. This is hence the received constellation before de-rotation and demapping. For this example, one may assume that only the I component undergoes fading, while the Q component is not attenuated. FIGS. 4A and 4B show the received constellations in the case of no fading. It is clear that both constellations provide the same capacity, such that constellation rotation and co-ordinate interleaving brings neither gain nor loss.
FIGS. 4C and 4D show the case, where either the complete constellation (or only its I component, respectively) is attenuated by 6 dB, i.e. the amplitude is halved. While there is a significant performance loss for the non-rotated case, as all Euclidian distances have been halved, the loss is lower in the rotated case. To be fair, one may not compare FIG. 4C immediately with FIG. 4D; in the former, both I and Q have undergone fading, while in the latter, it is only the I component; hence for FIG. 4D the channel appears to be much more favorable.
A fair comparison would be to consider two QPSK symbols, i.e. 2 I- and 2 Q-components. It is assumed that one cell, i.e. one I- and one Q-component, is affected by fading. For the non-rotated case, the attenuated I- and O-components are, of course, in the same mapped symbol, i.e. one has one attenuated symbol like in FIG. 4C and the other non-attenuated symbol looks like FIG. 4A. In the rotated case, one will have two mapped symbols like in FIG. 4D, where either only the I- or only the Q-component is attenuated.
As is commonly known, the non-rotated case with two differently attenuated symbols achieves only a smaller channel capacity than the rotated case with two similarly affected symbols, as the latter exploits a higher degree of diversity. The channel experienced by the rotated scheme (here, the “channel” includes the co-ordinate interleaving and de-interleaving) appears to be more “averaged” than the one experienced by the non-rotated scheme. As indicated by Jensen's inequality from information theory, an average channel has a higher capacity (for the same averaged signal-to-noise ratio, SNR) than averaging the capacity over different channels. This is the reason, why a (e.g. Rayleigh-) fading channel of a given mean SNR necessarily has a lower capacity than an AWGN of the same SNR.
The same principle applies here. The non-rotated case experiences one very good channel (FIG. 4A) and one rather poor channel (FIG. 4C), while the rotated scheme experiences twice a medium channel (FIG. 4D). In other words, the channel behavior has been averaged with respect to the non-rotated case. Therefore, the resulting symbols for the non-rotated case (lx like in FIG. 4A and 1× like in FIG. 4C) have a lower capacity than the corresponding symbols for the rotated case (2× like in FIG. 4D).
FIGS. 4E and 4F show the constellations for a very strong attenuation >10 dB. We see from FIG. 4E that the non-rotated symbol is practically useless, when the attenuation is strong, while the “half-way attenuated” rotated symbol merely degenerates to a kind of 4-ASK (Amplitude Shift Keying) constellation, which can be demapped quite well.
The conclusion is therefore that the co-ordinate interleaving, realized by the Cyclic Q delay and the Cell Interleaver in DVB-T2, increases the diversity order, thus averages the channel experienced by the rotated symbol (from mapper to demapper) and thus increases the channel capacity compared to the case of conventional non-rotated constellations.
Subsequently, the realization of the cyclic Q delay in DVB-T2 is discussed.
In DVB-T2, the Q components of all (QAM-) symbols belonging to a single FEC block (i.e. codeword) are shifted by 1 symbol with respect to their associated I component. That is, if the FEC block contains the following rotated symbols before the cyclic Q delay:
                    (                              I            0                    ,                      Q            0                          )                                (                              I            1                    ,                      Q            1                          )                                (                              I            2                    ,                      Q            2                          )                        …                                    (                                    I                              N                -                1                                      ,                          Q                              N                -                1                                              )                ,            where N is the number of symbols in the FEC block, then it will be the following so-called cells after the cyclic Q delay:
                    (                              I            0                    ,                      Q                          N              -              1                                      )                                (                              I            1                    ,                      Q            0                          )                                (                              I            2                    ,                      Q            1                          )                        …                                    (                                    I                              N                -                1                                      ,                          Q                              N                -                2                                              )                .            
The DVB-T2 standard states that the Cell Interleaver is a pseudo-random interleaver, which mixes up all the cells of a FEC block arbitrarily.
When the time interleaver is configured to provide interleaving over frame boundaries, then it divides a FEC block into several packets (let us refer to them as Interleaver Units (IUs) in the sequel). For time interleaving over M T2/NGH frames (each is, e.g., 200 ms long), the cells of the FEC block (as output by the Cell Interleaver) have to be partitioned into M IUs. These can have identical sizes, quasi-identical sizes (i.e. sizes differ at most by 1 due to rounding effects, because the FEC block length is not an integer multiple of M) or individual sizes.
Then the transmitter transmits one IU per frame, i.e. the M IUs (packets of cells) of one FEC block are transmitted in M (possibly subsequent) T2/NGH frames. An IU is hence a packet of cells, which (a) belong to the same FEC block and (b) are transmitted in the same T2/NGH frame.
Subsequently, the disadvantages of this approach are discussed with reference to FIGS. 5A-5C.
With the current T2 standard, the chain of Cyclic Q Delay, Cell Interleaver and Time Interleaver achieves a pseudo-random distribution of the I- and O-components of the rotated symbols composing one FEC block over the M T2 frames of the time interleaver duration. This leads to a situation as displayed in FIGS. 5A-5C. The time interleaver in this example is M=6 T2 frames long.
In FIG. 5A, the position of the I- and Q-components of various rotated symbols are displayed using the same indexing. For instance, the Q-component of the rotated symbol of index 0 is located in the top sub-carrier of frame 0, while the I-component of the same rotated symbol is located in the middle of frame 2. One can see that the pseudo-random distribution of the I- and O-components leads to cases, where both components of one rotated symbol are in the same frame (#2, #5), or where they are in different frames (all others).
Now, the frames consisting of these cells are transmitted over a time- and/or frequency-selective channel. In the example of FIG. 5B, the channel is frequency-flat but fading in time, as can be observed for the time-varying SNR. While frames 0, 3, 4, and 5 have quite high an SNR, frames 1 and 2 are received at poor SNR and are therefore more or less lost.
FIG. 5C then shows, which of the I- and O-components survive this fading channel. One finds that rotated symbols #0, #1 and #4 have one surviving component each, while #2 and #3 keep both components alive. On the other hand, #5 is completely lost.
As one saw before, this distribution of I- and Q-components is sub-optimum.
It is an objective of the present invention to provide an improved transmission or reception concept which has an increased robustness in non-optimum transmission conditions.